1) Field of the Invention
Embodiments of the present invention generally relate to a method of using an atomic force microscope (AFM), and more particularly to methods of acquiring amplitude versus distance measurement curves.
2) Description of Related Art
The surface properties of thin films can be measured by an atomic force microscope (AFM) operated in static or dynamic mode. In amplitude modulated (AM) dynamic mode, operation the AFM tip (probe) is typically vibrated or oscillated at a fixed frequency that is at or near the cantilever resonance frequency (reference). The amplitude and phase of the tip vibration is monitored by a detection mechanism as electronics controls the positioning of the tip relative to the sample surface. The measurable affect of the sample surface on the monitored probe tip is an output of AFM measurement.
Generally, as shown in FIG. 1, when an AFM tip gradually approaches a sample surface, the tip enters a van der Waal force field in a non-contact attractive force regime. Other attractive force fields, such as magnetic, capacitive, electric, friction, lateral, and capillary (mediated by a condensing vapor such as water) fields may also be measured with a tip so adapted. For clarity, the discussion of embodiments herein is focused on the van der Waal force, but it should be apparent that the present invention may be readily adapted to other attractive forces. Due to the attractive force field, the oscillating amplitude monotonically decreases with decreasing tip-to-sample distance. The phase signal also monotonically decreases (or increases, depending on the implementation of the phase detection electronics). Further reduction of the tip-to-sample distance will cause hard contact between the tip and the sample surface along with an abrupt and sharp reversal of the phase. The hard contact is first intermittent, occurring only during maximum tip displacement, but may become constant if the tip becomes embedded or otherwise captured by the sample surface.
During a non-contact surface scan, amplitude or phase modulation due to tip-to-sample interactions are maintained at a near constant value by a feedback control mechanism as an electrical or electromechanical apparatus scans the tip laterally across the surface. While scanning generally parallel to the sample surface, the feedback electronics are provided with an amplitude reference signal or a phase reference signal termed the feedback setpoint. The feedback mechanism attempts to maintain the actual tip oscillating amplitude or phase at the feedback setpoint by driving the cantilever vertically up and down in an effort to follow the contour of the sample surface topography and thereby maintain a constant tip-to-sample distance. It is desirable to operate this constant feedback setpoint at a value such that a pure non-contact attractive interaction between the tip and the sample is ensured, thus preventing the AFM tip from even momentarily making hard contact with the sample surface. Such hard contact usually causes AFM tip damage and results in loss of measurement accuracy and precision. Therefore, prior to scanning, a critical feedback setpoint where the onset of hard contact occurs should be determined precisely. The AFM will then typically be operated at either an amplitude setpoint or phase setpoint corresponding to a tip-to-sample distance that is nominally greater than the tip-to-sample distance corresponding to the critical phase setpoint or critical amplitude setpoint to ensure non-contact attractive interaction between the tip and the sample.
The range of tip vibration amplitude (or phase) that encompasses the onset of van der Waal (or other attractive force) interaction and the onset of hard contact depends on the initial free space vibration amplitude of the cantilever, the vibration frequency, tip material, sample material and other properties (such as stored electrical charges on insulating surfaces). The precise determination of this range will ensure that the AFM be operated in a non-contact (or attractive) regime and reduce the tip wear. In general, the most influencing factor is the cantilever vibration amplitude. At small vibration amplitude, this range, in a relative sense, can be large. While at large vibration amplitude, this range can shrink to zero and, when tip is close enough to the surface, it will bypass the van der Waal interaction and jump to hard contact almost immediately. It is also important to determine the change of vibration amplitude and phase as functions of absolute tip-to-samples distance in the van der Waal (or attractive force) regime, because such information provides valuable information of the interaction force between the tip and the sample. However, conventional method of obtaining both critical set point and functional relationships between amplitude/phase and absolute tip-to-sample distance frequently results in tip damage, as further described below.
Both FIG. 2a and FIG. 2b were obtained using the conventional method that caused the damage of AFM tip. Nevertheless, both figures still illustrate the transition from non-contact (attractive force) regime to hard contact regime of the broken tip unambiguously. FIG. 2a shows the AFM tip vibration amplitude versus relative tip-to-sample distance with an arbitrary 0 reference point. In the illustrated example, it can be seen that when the vibration amplitude is smaller than 3.4 nm, (correspondingly the relative tip-to-sample distance is smaller than 11 nm), the tip amplitude starts to decrease more rapidly with the reduction of tip-to-sample distance, signaling the beginning of the van der Waal interaction. The small deviation from the monotonic decreasing relationship occurs at the tip amplitude 0.4 nm. The arrow 202 in FIG. 2a points to the tip-to-sample distance where hard contact between the tip and the sample is thought to occur. An abrupt change in phase can also be used to determine the onset of intermittent hard contact between the tip and the sample. FIG. 2b illustrates the phase versus tip-to-sample distance relationship, and again, the arrow 212 in the figure points to where the onset of hard contact between the tip and the sample is thought to occur. Both curves can also be used to establish the functional relationships of amplitude and phase versus absolute tip-to-sample distance. The absolute zero tip-to-sample distance occurs at 6.4 nm in X scale in both graphs, where both amplitude and phase become 0.
Generally, the conventional approach to generate the curves shown in FIGS. 2A and 2B, is to: 1) perform AFM tip approach to the sample surface and stop tip approach at the proximity of sample surface; 2) with the feedback mechanism disabled, advance and retract tip repeatedly and record the amplitude and phase signals simultaneously; 3) establish the tip vibration characteristics corresponding to the point of surface contact as the critical feedback setpoint to be used during a subsequent scanning measurement and establish the functional relationships of amplitude and phase versus absolute tip-to-sample distance. Thus, this conventional method advances and retracts the AFM tip towards and away from the sample surface repeatedly without any feedback mechanism enabled while monitoring the amplitude and phase signals of the AFM tip vibration. From the resulting curves shown in FIG. 2A and FIG. 2B, the amplitude and phase signals corresponding to the point of contact are then assigned the critical feedback setpoints. However, during the conventional method, the tip is repeatedly rammed into the sample surface and the tip will likely be damaged. Due to tip damage, the critical setpoint established from the method may be grossly inaccurate as may be any subsequent measurements reliant on the F/D relationships deduced from the amplitude and phase measurements during the non-contact regime calibration. For example, when the feedback mechanism is enabled during subsequent AFM scanning operations, the critical setpoint employed as identified with the conventional method may have an unknown offset and the tip may be damaged in a manner reducing tip lifetime and/or measurement efficacy. Also, the functional relationships of amplitude and phase versus absolute tip-to-sample distance are those of the broken tip and the sample surface instead of the virgin non-damaged tip and sample surface.